The production of copolymers of 1-olefins is generally accomplished using the so-called Ziegler-Natta catalysts, among which combinations (mixed catalysts) of vanadium compounds and alkyl aluminum halides are best suitable. These compounds are soluble in the inert solvents in which polymerization is effected. One disadvantage is that the catalysts must be used in a relatively high concentration, since the amount of polymer formed per catalyst unit is not large. When attempting to overcome this drawback by an increased charge of monomers and/or by higher pressure, the catalyst activity is raised somewhat, but the molecular weight of the thus-formed polymers becomes so high that they can be processed only with difficulty. In addition, the relatively high molecular weight results in a high viscosity of the solution of polymers, so that heat removal during polymerization is made difficult; accordingly, polymerization in large reaction vessels becomes uneconomical.
It is also known that the yield of polymer can be increased by the addition of halogenated organic compounds, varyingly called promoters, reactivators or activators. Thus, in French Pat. No. 1,417,195, trihaloacetic acids, especially the esters thereof, hexachloropropylene, and hexachloroacetone are disclosed as promoters. Analogous use of hexachlorocyclopentadiene is known from French Pat. No. 1,370,358.
Such additives, however, are disadvantageous in that they inhibit the mixed catalysts when used in the concentrations which are necessary for the additives to be effective to an adequate extent.
These disadvantages have been diminished by the use of perchlorobutenoic acid compounds (German Pat. No. 1,595,442), prepared according to A. Roedig and P. Bernemann (Liebigs Ann. Chem. 600 : 1 [1956]). In this connection, it had previously been impossible to analytically determine whether compounds of perchlorocrotonic acid or of the isomeric perchlorovinylacetic acid were involved. Only in recent times has it been possible to unequivocally determine the structure as that of perchlorovinylacetic acid.
However, there has still remained a need for an improved process for producing copolymers of ethylene with at least one other 1-monoolefin and, optionally, an unconjugated diene.